In This Section

1-D ‘wires’ could advance quantum electronics

December 12th, 2017 ›

Electron microscope image of a 1-D channel
molybdenum disulfide embedded in 2-D tungsten selenide. The blue/green dots
are tungsten and selenium atoms; the magenta dots are sulfur atoms. The
entire film is three atoms thick.

Materials the thickness of a single
atom - known as 2-D materials - hold great promise for next-generation
electronics, but actually putting circuitry on something so thin is a
challenge.

"If I want to make an atom-sized
feature, and I don't have atom-size control, it's not going to work," said David Muller, the Samuel B. Eckert Professor of
Engineering in the Department of Applied and Engineering Physics.

Muller and his research
collaborators have discovered - a little bit by accident - a method for
basically inserting a 1-D semiconductor channel into the "fabric" of a 2-D
material. The electronic band structures of these channels exhibit the
properties necessary for future electronics applications.

Muller, co-director of the Kavli Institute
at Cornell For Nanoscale Science, said the seed for this work was
analysis of 2-D materials for the purpose of making computer circuits. In
examining the interface of two atomically thin materials - such as molybdenum
disulfide (MoS2) and tungsten selenide (WSe2) - Muller and Han noticed that the
atomic structures didn't line up perfectly, that there were defects where the
materials joined.

Muller likened the interface to
that of sheets of different thread counts.

"If I look at the two materials
with different thread counts, they don't match up when you put them together,"
he said. "If I line up my two sheets parallel, that's where you're going to get
the closest match. But what if I took the one sheet and I cut it at a 45-degree
angle? Now, when I rotate it to line up with the other one, the spacing between
the mismatches is going to be very different."

It's at these mismatches called
dislocations - the "loose threads" at the interface of the two materials -
where the 1-D wires form. The chemical reactivity of the materials is higher at
these defects than in the rest of the material, so when subjected to certain
growth conditions, the defects didn't just stay at the interface - they tended
to migrate.

"We were kind of surprised that the
little loose threads didn't stay at the interface," Muller said, "and what we
saw instead were these wonderful little wires."

The higher reactivity at the
dislocations allowed the molybdenum and sulfur atoms (Mo and S) effectively to
grow, away from the original interface. That had the effect of forming 1-D MoS2
"wires" in a trail behind the advancing core in the WSe2.

And by controlling the angle at
which the two materials are put together, "we can control how many 'loose
threads,' how many starting points, we have for growing these wires," Muller
said.

These 1-D wires could play a role
in future quantum electronics, Muller said.

"If I want to build a computer chip
at this scale, I need a couple of different components," he said. "I need a
good conductor, like [single-atom-thick] graphene, and I need a good
semiconductor. That would be these MoS2 's and WSe2' s."

Han said further work will look at
other materials, for which their computational collaborators at the
Massachusetts Institute of Technology already have predictions for future
candidate systems.

Researchers from Academica Sinica
(Taipei) also contributed to this work.

Electron microscopy for this work
was done at the Cornell Center for Materials Research, which is supported by
the National Science Foundation's Materials Research Science and Engineering
Centers program.

Support also came from the
Department of Defense Multidisciplinary University Research Initiatives and the
Office of Naval Research.